Electrical to mechanical magnetic transducer



May 24, 1960 H. BOWDITCH 2,933,151

ELECTRICAL TO MECHANICAL MAGNETIC TRANSDUCER Filed Sept. 12, 1958 2 Sheets-Sheet 1 SIGNAL IN ELECTRICAL IT) I PEN l MOTOR snsmu. our

MECHANICAL aug- INVENTOR HOEL L. BONDITCH AGENT May 24, 1 0 H. L. aowprrc 2,938,151

ELECTRICAL TO MECHANICAL MAGNETIC TRANSDUCER Filed Sept. 12. 1958 2 Sheets-Sheet 2 INVENTOR HOEL L.BOWDITCH AGENT United States Patent ce l 32322;

ELECTRICAL TO MECHANICAL MAGNETIC TRANSDUCER Hoel L. Bowditch, Foxboro, Mass., assignor to The Foxboro Company, Foxboro, Mass., a corporation of Massachusetts Filed Sept. 12, 1958, Ser. No. 760,662

1 Claim. (Cl. 317-123) This invention is in the field of industrial instrumentation, and is in the form of a device for producing a mechanical movement in representation of an electrical signal.

As an illustration, this invention may take the form of a. device for producing a recording instrument pen movement in response to an electrical signal derived from a variable condition value change.

The operation of the device of this invention is magnetic, and in the general nature of the dArsonval principre. Prior structures of this general nature have been low power devices or have been impractically bulky or inefiicient. 1

The device of this invention is a compact, powerful unit, which produces a substantial mechanical movement on a practical basis. Essentially, this device comprises a magnet, with an electrical coil bodily movable along the end face of one pole of the magnet, wherein the magnet 1.: wholly outside of the coil and the movement is produced by the interaction of the magnetic field of the magnet and a magnetic field set up by the application of an electrical signal to the coil.

It is therefore an object of this invention to provide a new and improved electrical to mechanical magnetic transducer.

Other objects and advantages of this invention will be in part apparent and in part pointed out hereinafter.

In the drawings:

Figure I is a schematic illustration of an application of a device according to this invention as a pen motor for an industrial instrumentation recording unit;

Figure II is a fragmentary illustration in perspective of an electrical coil movable along the face of one pole of a magnet, as the basis of the magnetic transducer action according to this invention;

Figure III is a front elevation of a structure embodying this invention;

Figure IV is a rear elevation of the structure of Figure III;

Figure V is a plan view of a section of the structure of Figure III, taken as on line VV in Figure III; and

Figures VI and VII are left and right endviews respectively of the structure of Figure III.

' In the Figure I schematic showing the device according to this'invention is indicated as a pen motor 10 to which an electrical signal is applied as indicated by the arrow 11. This signal may for example represent a value change in a variable condition of an industrial process. The output of the pen motor 10 is indicated by a dotted line 12, which represents mechanical movement as derived from the electrical input signals indicated by the arrow 11. Such'mechanical movement is applied to a recorder pen arm 13 to cause it to move about its pivot 14 to produce a record line 15 on a chart 16 which in this illustration is a strip chart moving downward in the direction indicated by arrow 17. Thus the pen motor 10 is an electrical tomechanical transducer.

Figure II illustrates the action of the transducer pen motor 10. This action is magnetic as between a permanent magnet 18 shown as presenting a single pole end face 19 and a flux guide number 20 which is in the form of a magnetic material strip lying spaced from and essentially parallel to the end face 19 of the permanent magnet 18 to provide an air and flux gap therebetween. Thus the flux from the magnet 18 passes through the air gap and into and along the flux guide number 20. An electrical coil 21 is mounted on the flux guide number 20 as a sleeve for movement therealong as indicated by the arrow 22. The coil 21 is provided with electrical leads 23 and 24 to which the electrical input signal 11 of Figure I is applied. Thus according to the dArsonval principle, when electrical current is applied to the coil 21, it is moved along the flux guide 20. It is this mechanical movement which results in the output of the pen motor 10 of Figure I and is taken off in suitable fashion as will be explained later herein. The part of the coil 21 which lies between the flux guide 20 and the magnet pole face 19 is the working part of the coil, that is, the part which is effective in a positive fashion to produce the desired action of this device. It should be noted that this part of the coil 21 is a large partof the total coil, and thus does not have to overcome an undesirably large resistance from a long wire length remainder of the coil. It should be noted as well that the magnet 18 and supporting arrangements for the coil 21 may together be mechanically designed to their best advantage with a minimum of consideration of the size and form of the coil 21. Thus for example the magnet 18 may be made large and heavy to produce a substantial amount of power without necessitating a comparable size change in the coil 21.

Figures III through VII are all related, as different views, to the pen motor structure and will thus be discussed together.

As seen in Figures III and IV as front and rear views of this structure, a permanent magnet 25 in bar form is provided, in a horizontal position. Surrounding this bar magnet in one plane direction is a flux guide member generally indicated at 26. This flux guide comprises a cross member 27 which is spaced from the end face of one pole of the magnet 25 to provide a fiux gap with respect thereto. The flux gap may be empirically provided with some width variance if desired, as a means of providing more uniform flux distribution. The remainder of the flux guide 26 is a U-shaped body 28 which is secured to the ends of the flux guide cross member 27 as by screws 29 and 30 at the mouth of the U.

The base of the, U member 28'is integrally secured to the opposite pole of the bar magnet 25 as at- 31. This may be accomplished, for example, by cementing the magnet into a guiding recess in the base of the U member. Thus the overall arrangement is that of a W with the outer legs of the W joined at their tops by the crosspiece 27 as a part of the flux guide member 26. Accordingly in this device a permanent magnet is provided with a single air gap 32 at one pole area, and the other pole area is directly secured to the flux'guide U member 28 at its base as at 31. Accordingly this device has a continuous flux guide path with the exception of the air and flux gap 31. I

With respect to the permanent magnet 25 at the left end thereof in Figure III, that is, the operating or flux gap end, a cap plate 33 is mounted thereon and provided with a convex outer surface as one face of the flux gap 32. This end plate is in itself a flux guide arrangement to spread the end flux of the magnet 25 and to provide an inexpensive end form for the magnet 25. The cross bar flux guide 27 is formed in an are essentially to match 3 the arc of the end plate 33, so that the flux gap 32 is essentially uniform in width from the magnet end plate 33 except as empirically varied to produce uniform flux distribution.

The magnet 25 of Figure III is comparable to magnet 18 of Figure II and the flux bar 27 of Figure III is comparable to the flux bar 26 of Figure II. Similarly, an electrical coil 34 is provided in Figure III and is comparable to the coil 21 as discussed in Figure II. The Figure III coil 34 is mounted as a sleeve on the flux bar 27 for movement therealong in the manner and for the purpose described for such a coil in Figure II. That is, the coil 34 in Figure III is arranged to have an electrical signal applied thereto, in which case the magnetic field of the coil 34 and the magnetic field from the magnet 25, react with each other in the flux gap 32 to move the coil 34 along the flux bar 27 in accordance with the electrical signal as applied to the coil 34.

The coil 34 is supported by a generally rectangular channel sided yoke 35 which may be seen in plan view in Figure V and one side of which is shown in Figure III at 36 and the other side in Figure IV at 37. The yoke 35 is made of lightweight aluminum, non-magnetic, and the sides 36 and 37 have a series of disc perforations therein to further reduce the weight. This yoke is pivoted as in Figure III at 38 which is an axis defined by the crossing points of a pair of sets of fiexures 39-40 and 41-42 (see Figures III and V). These fiexures are mounted between the base of the U shaped flux guide 26 and a counterweight body 43 mounted in the rectangular yoke 35 outwardly beyond the flexure axis 38, away from the magnet 25. Thus the yoke 35 has a long portion encompassing the magnet 25 and a short portion, in which the counterweight 43 is mounted.

As in Figure III, a return spring 44 is provided as a calibrated device in opposition to the electrical signal (D.C.) increase movement of the coil 34. The spring 44 is secured at one end to an adjustment assembly 45 and at the other end to a lever 46 which is in turn secured to the yoke 35 and the counterweight 43 as by screws 47. The adjustment assembly 45 includes an upper guide block 48 secured to the top leg of the flux guide U member 28 and a lower guide block 49 secured to the bottom leg of the flux guide U member 28. Mounted vertically in these guide blocks, and side by side, are a guide bar 59 and an adjustment bar 51. An adjustment block 52 rides vertically on both of these bars, slidingly on the guide bar 50 and by screw thread engagement on the adjustment bar 51. The return spring 44 is secured to adjustment block 52 and a bias spring 53 is provided on the adjustment bar 51 to take up the slack in the screw thread engagement of the adjustment block 52 with the adjustment bar 51. The top of the adjustment bar 51 extends above the overall device and is provided with an end screw driver slot 54 as a means of turning the adjustment bar 51 to adjust the spring 44.

Means for applying an electrical signal to the coil 34 (Figure III etc.) is provided in the form of a pair of electrical input connection posts 55 and 55'. Thus an electrical signal input, such as 11 in Figure II, may be applied to the connection posts 55 and 55" of Figure III etc. From these connection posts, electrical leads 56 and 56' are provided in the form of coil springs as electrical conductors. These springs are also connected into the arrangement of the yoke 35, as at 57 and 57. From these points, electrical leads 58 and 58' are taken, one along each of the side arms 36 and 37 of the yoke 35, to connect into the ends of the coil 34. This arrangement is schematically shown in Figure II as leads 23 and 24 leading to the coil 21. The Figure III and V leads 58 and 58 are not shown in their entirety since they procecd along the arms of the yoke 35 just under the top edge horizontal channel lips of these yoke arms.

Modern systems utilizing devices of this nature, for

example, means for operating a recording pen, require a powerful device with a substantial distance movement and with steadiness and reliability of operation. It is further desirable in many instances that the device be usable in areas where there are explosion hazards. Thus a slide wire arrangement is undesirable. Prior art systems are too delicate, low powered, or impractical for present needs. By using a single flux gap arrangement, a powerful magnet and a long lightweight yoke to locate the movable coil Wholly at one end of the magnet, the device of this invention provides a strong, compact system with long movement from a small pivot angle to directly translate a strong electrical signal into a strong mechanical movement. This arrangement thus lends itself readily to the use of a flexure pivot mount for the coil, thus providing a pivot which is shock-proof, wear-proof and unaffected by position change. For example in this device a direct current input may be of the order of ten to fifty milliamps. A recorder may have a load of the order of 200 ohms. This device then may be arranged to provide 500 milliwatts of power. The theoretical mechanical equivalent is four pounds acting through a distance of one inch per second.

Thus the magnet of this device may be massive, with high coercive force, high residual induction, and through proper manufacturing treatment of the magnet may be devised to continuously deliver a constant high level of flux in a single air gap. This power factor allows for ample clearance in the air gap for a big, rugged moving coil.

It should be noted that the moving coil is essentially equally divided with a large part inside of the main magnetic flux path as defined by the flux guide system, and something of the order of not more than a third of the coil outside of the main magnetic flux path. A large coil motion is obtained with this arrangement, of the order of one and three quarter inches. The coil and the mounting are essentially electrically and mechanically independent in that they provide a long support frame and a heavy magnet without necessitating enlargement of the coil since it is only the flux gap guide bar which the coil surrounds. The ratio of useful coil length to the total coil length is therefore large. As a further feature the angular motion of the coil is small, for example of the order of twenty degrees, and this enables the use of a flexure mounting for the coil as a pivot device. Further, with such a small angle of motion, the factor of possible angularity errors is negligible.

Thus this device provides, for example, a powerful pen motor which is simple and compact, in the nature of a direct current deflection motor, with a permanent magnet surrounded by a closed iron flux path except for a single flux gap, and with a flux path of extremely 'low reluctance.

In operation the coil is deflected against the action of the calibrated spring when the coils magnetic field interacts with the lines of force from the permanent magnet. Since the iron flux path is unbroken except for a minimum air gap there is a minimum flux leakage and consequent high power.

The frictionless fiexures provide a device which may be tilted or laid on its side or otherwise unusually treated.

The device of this invention is inherently an averaging device which with suitable associated circuitry (not shown) may be made to automatically damp violent oscillations.

In the system according to this invention the force on a conductor perpendicular to a magnetic field is directly proportional to the length of the conductor in the field, to the current flowing in the conductor, and to the flux density in the field. With the device of this invention there is a substantial length of conductor in the field, the signal current applied to the conductor can also be substantial, and the flux density in the field is made substantial by the fact that the design and arrangement of this device allows the use of the large and powerful magnet.

The pen motor of this device is inherently a self temperature compensating device. This is accomplished by the fact that changes in temperature affect the permeability of the magnet and also the tension in the compensating or return spring. When subjected to a rise in temperature the permeability of the magnet material decreases so that the magnet produces less flux density in the air gap. Simultaneously the spring is weakened by the temperature rise. Thus effective temperature compensation can be achieved by proper selection of spring and magnet materials in opposing action direction in terms of temperature effect.

This invention provides a sleeve coil on a transverse flux bar. Accordingly a large work factor is made possible. The magnet may be made large, the flux gap substantial, and the moving coil may be relatively large, with a large portion of its windings within the flux gap. On this same basis a long coil movement over a small pivot angle is made possible, and accordingly, the desirable flexure pivot mounting of the coil can be used.

With the power provided in this device sufficient weight may be incorporated to give good stiffness in rugged linkages which may be operated through strong pivot or adjustment arrangements, and devices such as damping dashpots may be used. Thus a positive and rugged pen drive action may be provided in a recording instrument.

This invention lends itself to the arrangement of a magnet with a single fiux gap at one pole only.

This invention, therefore, provides a new and improved transducer device for changing an electrical signal into a mechanical motion on a magnetic basis.

As many embodiments may be made of the above invention and as changes may be made in the embodiment set forth above without departing from the scope of the invention, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative only and not in a limiting sense.

I claim:

An industrial instrumentation recording pen motor device in the form of an electrical to mechanical transducer for receiving intelligence with respect to the value of a variable condition in electrical signal form and for producing therefrom a mechanical movement as the basis of a recording pen movement in representation of said variable condition value intelligence, said device comprising a U-shaped flux guide member, a bar magnet lying within the U shape of said flux guide member with one end secured to the base of said U member as the only contact of said bar with said U-shaped flux guide member, a flux guide cap on the other end of said bar magnet, with said cap extending across and somewhat beyond the end face of said other end of said bar magnet and having a convex outer surface, a transverse flux guide member joining the ends of the legs of said U member and lying in spaced essentially concentric relation with said convex outer surface of said magnet end cap to provide a flux gap therebetween, an electrical coil mounted as a sleeve on said transverse flux guide member for movement therealong, said mounting comprising a frame about said magnet, a flexure pivot supporting said frame on the base of said U member, and a counter-balance weight on said frame beyond said pivot away from said magnet, a return spring mounted to oppose said movement of said coil in one direction, means for applying an electrical signal to said coil whereby the combined magnetic effects of said coil and said magnet produce said coil movement along said transverse flux guide member in representation of said electrical signal input, said last named means comprising a pair of fixedly mounted electrical lead connection posts, a pair of coil spring electrical leads from said connection posts to connection points on said pivoted frame, and a pair of leads from said connection points along said frame to said movable coil, and means for taking off said coil movement for application to a recording pen, comprising a lever fixed to said frame for movement about the axis of said fiexure pivot.

References Cited in the file of this patent UNITED STATES PATENTS 786,696 Vreeland Apr. 4, 1905 885,545 Thomas et a1. Apr. 21, 1908 2,798,972 Booth July 9, 1957 

